I pulled on that cable your are talking about, it didnt have any play and was well planted!! Except on mine, it doesnt go to the firewall, its right on the frame/body of the car along with a bunch of other grounds. Looked solid with no green cancer.
Im thinking im going to re do all the ground cables with a heavier gauge wire, since all the wires on this car as super thin, and if i decided to use an MSD coil or any other higher gauge plug wires, id be affraid of melting them for one thing and secondly it would be a pointless mod in the end...ill replace the postive batt. lead too.